Concrete Calculator

Work out how much concrete you need for a slab, footing, post hole or set of steps — in cubic yards, and in the number of 40, 60 or 80 lb bags to buy. Add several pours to one order and the total updates as you type.

Dimensions

Length

Width

Thickness

Quantity

Extra for waste

Bag size

Price per bag

Ready-mix per yd³

Concrete needed

1.36cubic yards

36.67 cu ft · 1.04 m³ · includes 10% extra

Between one and two yards, compare a short-load ready-mix delivery with 62 bags. Mixing that many bags by hand is a long day.

Premixed bags

40 lb bags (0.3 cu ft each): 123
50 lb bags (0.375 cu ft each): 98
60 lb bags (0.45 cu ft each): 82
80 lb bags (0.6 cu ft each): 62

Cost estimate

62 × 80 lb bags: $403.00
Ready-mix, 1.5 yd³ ordered: $262.50
Wet weight: 5,500 lb (2.75 tons)

How to use the concrete calculator

Pick the shape: Slab for patios, pads and floors, Footing / wall for continuous footings, curbs and poured walls, Column / hole for post holes and round forming tubes, and Steps for a stair run.
Type each dimension and choose its unit. You can mix units freely — length in feet, thickness in inches, or everything in meters and centimeters.
Use Add another pour when one order covers several parts, such as a slab plus its thickened edge, or a row of deck footings.
Keep the 10% waste allowance unless your forms are exact. Enter your local bag and ready-mix prices to compare the two ways of buying.

The concrete volume formula

Every shape comes down to cubic feet first, then cubic yards, because US ready-mix is sold by the cubic yard and one cubic yard is 3 × 3 × 3 = 27 cubic feet.

Slab or footing: length × width × thickness (all in feet) ÷ 27.
Round column or hole: π × (diameter ÷ 2)² × height ÷ 27.
Steps: width × run × rise × n(n+1) ÷ 2, where n is the number of steps. Each step is a solid block reaching down to the ground, so the bottom step is one riser tall, the next two, and so on.

Why divide by 324?

Thickness is almost always measured in inches while length and width are in feet. Converting inches to feet means dividing by 12, and converting cubic feet to cubic yards means dividing by 27. Doing both at once is dividing by 12 × 27 = 324. So square feet × inches ÷ 324 = cubic yards: a 400 sq ft slab at 4 inches is 400 × 4 ÷ 324 = 4.94 cubic yards.

How many bags of concrete do I need?

Divide the volume in cubic feet by the yield printed on the bag, and round up. Standard premix yields 0.30 cu ft per 40 lb bag, 0.45 cu ft per 60 lb bag and 0.60 cu ft per 80 lb bag. One cubic yard therefore takes 90, 60 or 45 bags. The 80 lb bag is the cheapest per cubic foot; the 60 lb bag is easier to lift and mix in a wheelbarrow, which matters more than price once a job runs past forty or fifty bags.

Worked examples

Example 1 — a 12 × 16 ft patio, 4 inches thick

12 × 16 = 192 sq ft. 192 × 4 ÷ 324 = 2.37 cubic yards (64 cu ft). With 10% extra that is 2.61 yd, so you would order 2.75 yards of ready-mix. In bags it is 118 × 80 lb or 157 × 60 lb — at that size, ready-mix is clearly the better choice.

Example 2 — eight deck footings in 10-inch tubes, 42 inches deep

One tube: π × (5 ÷ 12)² × 3.5 = 1.91 cu ft. Eight tubes: 15.27 cu ft, or 0.57 cubic yards. With 10% extra you need 16.8 cu ft: 38 bags of 60 lb or 28 bags of 80 lb. That is well under a yard, so bags are the obvious choice.

Concrete for common slab sizes (4 inches thick)

No waste allowance included. For other thicknesses, scale linearly: a 6-inch slab needs 1.5 times the 4-inch amount.

Slab	Cubic yards	40 lb bags	60 lb bags	80 lb bags
4 × 4 ft	0.2	18	12	9
6 × 6 ft	0.44	40	27	20
8 × 8 ft	0.79	72	48	36
10 × 10 ft	1.23	112	75	56
10 × 12 ft	1.48	134	89	67
12 × 12 ft	1.78	160	107	80
12 × 16 ft	2.37	214	143	107
12 × 20 ft	2.96	267	178	134
16 × 20 ft	3.95	356	238	178
20 × 20 ft	4.94	445	297	223
24 × 24 ft	7.11	640	427	320

Cubic yards by area and thickness

Area	3.5" deep	4" deep	5" deep	6" deep	8" deep
50 sq ft	0.54 yd³	0.62 yd³	0.77 yd³	0.93 yd³	1.23 yd³
100 sq ft	1.08 yd³	1.23 yd³	1.54 yd³	1.85 yd³	2.47 yd³
200 sq ft	2.16 yd³	2.47 yd³	3.09 yd³	3.7 yd³	4.94 yd³
300 sq ft	3.24 yd³	3.7 yd³	4.63 yd³	5.56 yd³	7.41 yd³
400 sq ft	4.32 yd³	4.94 yd³	6.17 yd³	7.41 yd³	9.88 yd³
500 sq ft	5.4 yd³	6.17 yd³	7.72 yd³	9.26 yd³	12.35 yd³
600 sq ft	6.48 yd³	7.41 yd³	9.26 yd³	11.11 yd³	14.81 yd³
800 sq ft	8.64 yd³	9.88 yd³	12.35 yd³	14.81 yd³	19.75 yd³
1,000 sq ft	10.8 yd³	12.35 yd³	15.43 yd³	18.52 yd³	24.69 yd³

How thick should the concrete be?

Project	Depth	Notes
Sidewalk, path	4 in	Cut control joints every 4–5 ft.
Patio	4 in	Slope 1/8–1/4 in per foot away from the house.
Shed or hot-tub pad	4–6 in	6 in under a filled hot tub (about 4,000+ lb).
Driveway	4–6 in	5–6 in where trucks or heavy SUVs park.
Garage floor	4–6 in	6 in for RVs, boats on trailers or a car lift.
Continuous footing	8–12 in deep	Width about twice the wall thickness; below frost line.
Deck / post footing	8–12 in diameter	Depth set by your local frost line, often 36–48 in.
Steps	7 in rise, 11 in run	Typical residential code: max 7¾ in rise, min 10 in run.

Concrete and base material weights

The weight of the concrete matters for hauling and for anything you pour over a structure. The aggregate figures are for the gravel base that usually goes under a slab; the gravel calculator works out tons for any area.

Material	lb / cu ft	lb / cu yd	tons / cu yd	Typical use
Gravel (general, 3/4")	104	2,800	1.4	Driveways, drainage, sub-base
Pea gravel	100	2,700	1.35	Paths, play areas, between pavers
Crushed stone (#57)	100	2,700	1.35	Drainage, concrete sub-base
Crusher run / road base	111	3,000	1.5	Compactable base for driveways and pavers
River rock	100	2,700	1.35	Decorative beds, dry creek beds
Decomposed granite	100	2,700	1.35	Paths and patios, compacts firm
Sand (dry)	100	2,700	1.35	Paver bedding, leveling, play sand
Topsoil	81	2,200	1.1	Lawns, raised beds, grading
Fill dirt	74	2,000	1	Filling holes and low spots
Compost	37	1,000	0.5	Soil amendment, top dressing
Mulch (shredded hardwood)	22	600	0.3	Flower beds, around trees
Wood chips / playground chips	19	500	0.25	Paths, playgrounds
Concrete (wet, normal weight)	150	4,050	2.03	150 lb per cubic foot
Asphalt (hot mix, compacted)	145	3,915	1.96	145 lb per cubic foot

Ordering for a slab rather than a footing? The concrete slab calculator adds a rebar grid and the gravel sub-base to the same estimate.

Frequently asked questions

How many 80 lb bags of concrete make a cubic yard?

An 80 lb bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so it takes 45 bags of 80 lb mix to make one cubic yard. With 60 lb bags (0.45 cu ft) it takes 60 bags, and with 40 lb bags (0.30 cu ft) it takes 90.

How do I calculate cubic yards of concrete?

Multiply length × width × thickness with all three in feet to get cubic feet, then divide by 27. If the thickness is in inches, multiply length × width in feet by the thickness in inches and divide by 324. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 4 ÷ 324 = 1.23 cubic yards.

How much extra concrete should I order?

Add 5–10% for spillage, uneven subgrade and form bulge. Use 10% for slabs on dirt you graded yourself and for anything poured into holes, where the true size is hard to measure; 5% is enough for formed walls and footings with flat, compacted bottoms. Running short in the middle of a pour leaves a cold joint, which costs far more than a few extra bags.

Is it cheaper to use bags or ready-mix concrete?

Below about one cubic yard, bags are almost always cheaper because ready-mix plants charge a short-load fee on small orders. Between one and two yards the two are close and the deciding factor is labor: two cubic yards is 90 bags of 80 lb mix. Above two yards ready-mix wins on both cost and quality, because the whole pour is one consistent batch.

How thick should a concrete slab be?

Four inches is standard for patios, sidewalks and shed floors. Use 5–6 inches for driveways and garage floors that carry cars, and 6 inches or more for RVs, boats or heavy trucks. The slab calculator page covers thickness, rebar and gravel base in more detail.

How much does a yard of concrete weigh?

Normal-weight concrete weighs about 150 lb per cubic foot when wet, which is roughly 4,050 lb, or just over two tons, per cubic yard. That is why a full ready-mix truck carries only about 8–10 yards, and why a single cubic yard is more than most half-ton pickups are rated to carry.